1. Technical Field
The present invention relates to an audio-video device for processing received transmitted signals, such as broadcast signals for a television and, in particular, to a method of controlling operation of the audio-video device during the time the device is in the possession of the end user.
2. Description of the Related Art
A wide variety of techniques for broadcast transmission are known in which the broadcast signal is encoded, scrambled or encrypted in some way to allow only authorized recipients to retrieve the original signal. One particular field in which this area has been researched is broadcast television.
The broadcast of television signals in which only permitted or authorized recipients can produce the clear television picture from those signals is known as Conditional Access Television or Pay-TV. In this context, broadcast can include over-air, via satellite, by cable or indeed any appropriate distribution medium in which the same signal content is sent to many recipients. Television signals may be analog signals or digital signals. The term “scrambling” is often used for the process of rendering analog signals unusable until “descrambled”, whereas the terms “encryption” and “decryption” are more often used for digital signals. In either case, the aim is to only allow users that have paid a subscription to descramble or decrypt the signals. The terms decoder and decoding will be used herein to cover both analog and digital devices and processes.
A known system and receiver for processing received signals is described in EP 0,428,252 which is illustrated in FIG. 1. The concept in this system is to broadcast signals in the air (by satellite) which can be received by anyone but only rendered usable by recipients having a “set-top box” decoder 2 and an associated smart card 22. The decoders 2 of all recipients are identical, but the smart cards 22 contain unique encrypted or secret information, including entitlements, which specify the channels within the broadcast signals the user is permitted to watch. The system operates broadly as follows:
A television signal is broadcast over air in a scrambled form and includes a stream of control data describing how the television signal is to be descrambled. The television signals and control data are necessarily the same signal sent to all users. It is not feasible to send the signals uniquely scrambled/encrypted to each recipient as there may be tens of millions of users and this would require tens of millions of times the bandwidth. Accordingly, all recipients must be able to operate the same descrambling/decryption process. This is implemented in the decoder 2, which receives the broadcast signals from a receiver 12. A data demodulator 14 extracts the portion of the signal for picture and/or sound and provides this to a descrambler 16 for descrambling. The control data portion is extracted and provided to a verifier 20 over the line 15. The control data comprises encrypted control words, which are needed to instruct the descrambler how to descramble the picture/sound signal. The control words must therefore be decrypted, and it is for this purpose that the smart card 22 is provided.
The verifier 20 provides encrypted control words across an interface along line 21 to the smart card 22. The smart card 22 contains an algorithm, which, if the user is entitled to watch the chosen channel, decrypts the control words and provides them to the verifier 20 via line 23. The verifier passes the decrypted control words to a PRBS 18, which in turn provides a descrambling code to the descrambler. It should be noted that the control words and hence the descrambling code change frequently (every few seconds). The security in this arrangement is thus that it is not feasible to try and decrypt the control words in real time without the smart card algorithm. Also, in the event that the smart card algorithm is compromised, then the smart cards themselves can be re-issued to all subscribers. Lastly, to view any channels, a user must pay for “entitlements” which are broadcast the over air and are addressed uniquely to each user and stored in the smart card 22.
A second published system is disclosed in a paper “Security and Addressability for Pay-TV” given at The Video Revolution Conference July 1982, University of Reading. In this system, it is proposed that a monthly key is broadcast to each subscriber using each subscriber's unique unit key stored in a decoder. In turn the monthly key, which is common to all users of the system is used to decrypt a program key for decrypting a given television program.